Phosphorylation by GSK-3β increases the stability of SIRT6 to alleviate TGF-β-induced fibrotic response in renal tubular cells

Aims: The deacetylase Sirtuin 6 (SIRT6) is up-regulated during fibrogenesis in renal tubular cells and post-ischemia/reperfusion kidneys. Hence, our aim was to investigate the mechanism of SIRT6 up-regulation upon profibrotic stress. Main methods: Immunohistochemical staining was used to detect the expression of UBC9 in the kidney section. The interaction of GSK-3β and SIRT6, and phosphorylation level of SIRT6 were detected by the immunoprecipitation assay. The wild-type and phosphorylated site mutant plasmids of SIRT6 were constructed and stably transfected to BUMPT cells to evaluate the phosphorylation function of SIRT6 by immunoblotting assay. Key findings: The phosphorylation of SIRT6 is significantly increased during TGF-β treatment in mouse renal tubular cells. GSK-3β can physically interact with SIRT6 in renal tubular cells, and this interaction is enhanced by TGF-β treatment. Moreover, GSK-3β is the phosphorylation kinase for SIRT6, and phosphorylates SIRT6 at Serine 326 residue to prevent its ubiquitination-mediated proteasomal degradation. Non-phosphorylatable mutant, S326A, of SIRT6, restores β-catenin activation and fibrotic changes in renal tubular cells. Significance: The present study demonstrates that a new mechanism for GSK-3β-mediated anti-fibrotic function in renal fibrosis through phosphorylation of SIRT6 to prevent its proteasomal degradation.